1. Field of the Invention
The present invention relates to a DC-DC converter circuit in which the primary winding side and the secondary winding side of a transformer are insulated from each other, and the primary side of the DC-DC converter circuit has a circuit for correcting the input voltage.
Priority is claimed on Japanese Patent Application No. 2002-365191, filed Dec. 17, 2002, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 3 shows a conventional DC-DC converter circuit. In the figure, reference numeral 1 indicates a transformer, reference numeral 2 indicates a main switching element, reference numeral 7 indicates an auxiliary output voltage circuit for smoothing the voltage at an auxiliary winding of the primary winding side of the transformer 1, and reference numeral 105 indicates a voltage measuring circuit. That is, this DC-DC converter circuit has a switching circuit which includes the main switching element 2 serially connected to the primary winding of the transformer 1, in which the primary winding side and the secondary winding side are insulated from each other. In this DC-DC converter circuit, the output voltage of the secondary side (i.e., the output side) is stabilized by performing PWM (pulse width modulation) control of the main switching element 2. Additionally, in order to indirectly measure the voltage of the secondary side at the primary side, the voltage measuring circuit 105 is provided for measuring the output voltage of the auxiliary output voltage circuit 7, which is output by smoothing the auxiliary winding voltage of the transformer 1, where the auxiliary winding voltage is in proportion to the output voltage of the secondary side.
However, in such an indirect measurement by measuring a state of the primary side, the measured level varies, particularly according to variation in the input voltage (i.e., Vin), so that an error is produced between the measured state and the actual state at the secondary side, thereby requiring an additional correction circuit. If variation in the measured level is in inverse proportion to the input voltage, an inverter circuit for inverting the measured input voltage value (obtained by resistance voltage division of the input voltage, or the like) is required as the auxiliary circuit. Such an inverter circuit is a complicated circuit employing a semiconductor which functions as an operational amplifier or the like, thereby producing problems such that the manufacturing cost is high, the area necessary for mounting such a complicated circuit is large, and the like.
On the other hand, in a known correction method used for a power supply which performs stabilization control of the output voltage of the secondary side by controlling the current at the primary side of the transformer, the limiting value for the output current at the secondary side is corrected so that the value is fixed, independent of variation in the input voltage at the primary side. More specifically, the input voltage is divided and the divided voltage is varied by a variable voltage generation circuit, and this voltage is provided as a comparative reference voltage to a comparison circuit, so that a control variable, which is in inverse proportion to the input voltage, is obtained so as to correct the measured current value (refer to Japanese Unexamined Patent Application, First Publication No. Hei 11-252908 (see FIGS. 1 and 2), etc.).
In the above-explained DC-DC converter circuit, the structure of the variable voltage generation circuit is complicated and a comparative circuit is necessary; thus, the number of necessary parts is large.